This invention relates generally to the heating of products with infrared (IR) energy, and more particularly to a gas-fired IR heater for projecting an IR beam in a radiation pattern having a predetermined geometry for irradiating the surface of the product to effect uniform heating thereof at a rapid rate.
The transfer of heat takes place by three processes: conduction, convection and radiation. In conduction, heat is transferred through a body by the short range interaction of molecules and/or electrons. Convection involves the transfer of heat by the combined mechanisms of fluid mixing and conduction. In radiation, electromagnetic energy is emitted toward a body and the energy incident thereto is absorbed by the body to raise its temperature. Radiant heating therefore differs from both convection and conduction heating, for the presence of matter is not required for the transmission of radiant energy.
According to the Stefan-Boltzmann law, the rate of heat transfer between a source of radiated heat whose temperature is T.sub.s and an absorbing body whose temperature is T.sub.b is equal to T.sub.x.sup.4 -T.sub.b.sup.4 ; that is, to the difference between the fourth powers of these temperature values. In convection heating, the rate of heat transfer is proportional only to the temperature difference between the body being heated and the surrounding atmosphere. Hence convection heating is inherently very slow, as compared to the nearly instantaneous effects of radiant heating.
Though an IR heater in accordance with the invention may be used throughout the full range of heating applications, including industrial processes such as industrial finishing and textile treatment, as well as in annealing, curing and drying operations which require heating, it will mainly be described herein in connection with the heating of food products; for the invention has particular advantages in that context.
While a food product typically undergoes cooking or baking at a temperature in the range of about 140.degree. to 200.degree. F. whose upper value is below the boiling point of water (212.degree. F.), it is nevertheless necessary in a conventional convection oven to establish a much higher oven temperature--usually well over 400.degree. F. The reason for this requirement is that the transfer of heat between the hot atmosphere of the convection oven and the body of food takes place at a fairly rapid rate only when the temperature differential therebetween is great.
If, therefore, the food placed in an oven is initially at room temperature and the oven temperature is held at about 200.degree. F., then as the body of the food becomes warmer and its surface temperature rises to, say, 150.degree. F., the rate of heat transfer as the temperature differential narrows thereafter becomes increasingly slow, and the cooking or baking process is protracted. On the other hand, if the oven temperature is raised to 400.degree. or 500.degree. F. to speed up baking, this means that the entire volume of air in the oven must be at this elevated temperature, and this entails a relatively large energy expenditure. With rising energy costs, this factor adds substantially to the cost of baking and is reflected in the cost of the product to the consumer. Also, with convection ovens, the flow of hot air over the surface of the food product tends to deprive it of moisture and volatile constituents and therefore degrades the quality of the product.
Radiation heaters in present commercial use are of the infrared type, the infrared band of thermal radiation lying within the electromagnetic wave spectrum. The quality and intensity of radiation in the infrared band of 0.7 microns to 400 microns depends on the temperature of the radiating body. If, therefore, the radiating body is a refractory ceramic heated by a gas-fired jet burner, one can only accurately adjust the quality and intensity of the IR radiation if it is possible to carefully control the operation of the gas-fired burner.
Despite the fact that IR heaters are much more economical to operate and act with extreme rapidity, and IR heaters are therefore far superior in this regard to convection ovens for cooking or baking food, they have enjoyed limited success in the baking industry. The reason for this is that commercially available gas-fired IR heaters are relatively difficult to control and also give rise to an uneven baking action.
Effective infrared heating depends not only on the radiant source temperature but also on what is referred to as the "geometric view factor." This factor determines the relationship between the pattern of IR radiation and the surface of the product being heated. With the typical IR heating arrangement, portions of the product to be heated are more completely exposed to IR rays and will be heated more rapidly to a high temperature than those portions that are not as fully exposed. As a consequence, the product may not be properly baked and may not be commercially saleable.
This drawback of IR heating with existing equipment is recognized in the article "Radiant Convection Heating--A Marriage of Two Systems" by H. J. Bennett, which appears in the journal Industrial Gas for February 1976. In order to overcome the uneven heating experienced with IR heating, the author proposes combining an IR heater with a convection heater so as to provide a heating technique somewhat faster than convection heating, yet with the uniformity and controlled temperature characteristics of convection heating.
The fact is, however, that the synthesis of IR and convection heating represents a compromise that is not entirely satisfactory, for it requires much more energy than IR heating and also a confined oven as well as separate controls for the heater and the over.
Ideally, with a food product, such as dough to be baked, having an exposed surface of given dimensions, the geometry of the IR beam impinging on this surface should be such as to impinge on all points thereon IR rays of equal intensity so that the baking is uniform throughout the body of the food. But existing IR heaters are incapable of producing an IR radiation pattern of uniform flux density which is so shaped as to uniformly irradiate and heat a given food product.